The cotton bollworm, Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae), is a significant cotton pest worldwide. Bacillus thuringiensis (Bt) cotton producing Cry1Ac has been used since 1997 for the control of this pest in China and a significant increase in H. armigera resistance to Cry1Ac has occurred in northern China. To mitigate resistance evolution, it is necessary to develop and plant pyramided 2- and 3-toxin Bt cotton to replace Cry1Ac cotton. For sustainable use of pyramided Bt cotton, we used diet overlay bioassays to measure the baseline susceptibility of H. armigera to Cry2Ab in 33 populations collected in 2017, 2018, and 2021 in 12 locations from major cotton-producing areas of China.The lethal concentration killing 50% (LC₅₀) or 99% (LC₉₉) of individuals from the populations ranged from 0.030 to 0.138 µg/cm² and 0.365 to 2.964 µg/cm², respectively. The ratio of the LC₅₀ for the most resistant and susceptible population was 4.6, indicating moderate among-population variability in resistance.The susceptibility of H. armigera to Cry2Ab did not vary significantly over years. A diagnostic concentration of 2 µg/cm² was calculated as twice the LC₉₉ from an analysis of pooled data for the field-collected populations. This concentration discriminated well between susceptible and resistant individuals, as it killed all larvae from a susceptible laboratory strain and 0%, 0%, and 23% of larvae from 3 laboratory strains with > 100-fold resistance to Cry2Ab.These baseline susceptibility data and diagnostic concentration for Cry2Ab will be useful for monitoring the evolution of H. armigera resistance to pyramided Bt cotton in China.